A context reference for the present invention is Pressure-Balanced Rod Piston Control System for a Subsurface Safety Valve U.S. Pat. No. 6,173,785 B1. The operating mechanism for the isolation valve for subsea use will be very similar to that which is shown in FIGS. 1 and 2 of this patent.
In fact, the pressure balance functionality of the operating mechanism unchanged; however, with the present invention the annular space surrounding the operating mechanism will be fully isolated from exposure to production fluids and solids (asphaltenes, paraffins, scale, etc.) and instead filled completely with a clean lubricating fluid increasing the service life of all moving parts. Isolation is enabled by integrating annular seals about the flow tube at the top and bottom end of the surrounding annular space. The annular space will be pressure compensated to internal tubing pressure to eliminate the buildup of pressure differential across the flow tube. In doing so the seal friction associated with the seals about the flow tube, which is exacerbated by increased pressure differential (thereby energizing the seals), will be minimized and will thus result in lower hydraulic control line requirements (i.e. operating pressures) for the valve. Further a safety valve designed for borehole applications is adapted for applications requiring resistance to tension, compression and bending loads, external exposure to subsea environments, and high pressure/high temperature service by utilizing a uniquely configured flanged outer housing into which the borehole safety valve is inserted, rotationally aligned for optimal closure mechanism performance, and secured with hydraulic control lines through the housing aligned and sealed to the control line connections on the borehole safety valve.
Traditional flapper style subsurface safety valves are known to suffer from debris related issues in downhole production environments. These issues can come in various forms and can range from mechanical fouling, caused by the accumulation of sand in the annular spring cavity, to piston or flow tube sticking caused by the solidification of heavy asphaltene or paraffin formations in the same region. Issues such as these can be extremely costly to operators given that their mitigation traditionally requires an intervention, at a minimum, and often times a full work-over to pull and replace the valve and/or completion. In the context of isolation valves for use within subsea production environments (i.e. installations within subsea production risers), the threat of debris fouling is exponentially increased. This is primarily attributable to the decrease in temperature and pressure of the production fluids as they travel through long stretches of Production Risers normally exposed to the cold marine environments. Consequently, waxes (paraffins) and other organic solids (such as asphaltenes) are known to precipitate out of production fluids in much higher rates and concentrations in these conditions. This debris problem requires routine remedial activity in the form of round trip “pigging” to scrape the I.D. of the Riser and clear solid deposits from its flow path.
Many subsea isolation valve installations such as those involving a semisubmersible rig use hydraulically operated plug or ball valves coupled to large hydraulic operators for quarter turn operation between open and closed positions. Because of the high operating pressures and the line sizes involved these valves are extremely heavy and have a fairly large profile exposed to wave and underwater current action. As a result such weight must be offset with buoyancy and the fixation system for the rig has to resist the wave and subsea current loading against the large profiles of these valves and their operators.
To address these issues the present invention proposes to use subsurface safety valves (SSV) of the type well known in the art in place of the heavy and high profile valves now being used as subsea isolation valves. To do this adaptations are made to enable a functionality akin to the traditional and proven operation of a subsurface safety valve yet in a marine environment, now in the horizontal orientation (in the preferred embodiment) and subject to bending loads (acting perpendicular to the longitudinal axis of the valve) resulting from wave action and subsea currents across the valve body. The borehole safety valves are adapted for the new environment with an outer housing and a sealing system that allows the borehole safety valve to be inserted in the housing and secured in a proper alignment with passages in the safety valve and surrounding housing sealingly aligned with oppositely oriented split sealing rings. The use of the outer housing and sealing system enables the effective implementation of traditional subsurface safety valve or subsurface barrier valve technologies in the marine environment,
Apart from the above reference, the following references illustrate the state of the SSV art and position indication art: U.S. Pat. No. 8,176,975; US 20060157240; U.S. Pat. No. 7,533,693; U.S. Pat. No. 3,077,179; U.S. Pat. No. 5,890,450 and US 20120234410. Pressure compensation systems for downhole use are shown in U.S. Pat. No. 6,041,857 in the context of a rotationally operated downhole choke valve.
Apart from using an SSV as a subsea isolation valve, other novel features are the ability to put an external position indication to that valve and the manner in which such an external indication system operates. The hydraulic system operates the local indication feature. The hydraulic system normally has discrete lines leading to opposed ends of an operating piston for the SSV. Movement of the operating piston moves a flow tube against a flapper for the open position. A closure spring closes the valve by pushing up the flow tube when pressure is released in the operating control line. The force closed line goes to the back side of the operating piston and can be used to force the operating piston back up to close the valve when the closure spring fails to do so. Local hydraulic taps into those two control lines provide for local operation by a diver or an ROV. The local position indicator is preferably integrated into the force close line for the main reason that such line operates at significantly lower pressures than the pressures seen in the operating control line. The position indicator is a volumetric displacement device. The volume displaced by the operating piston is the volume pushed to the position indicator. Depending on the relative diameters of the operating piston compared to the indicating piston the movement of the indicating piston will be the same or different than the movement of the operating piston. In this manner the application of the known SSV design to a subsea application allows it to meet the requirements of API-6A/6DSS/17D standards. The indicator mechanism can be configured to be depth insensitive to seawater. The indicator can be calibrated as part of the manufacturing and assembly process in conjunction with its associated valve to compensate for any trapped compressible fluids in the system. The indicator piston can displace trapped fluid against a floating piston to isolate the indicating piston and its seals from seawater. The pressure compensation system for a flow tube and the sealing connections to an outer housing for high pressure subsea application and other features of the present invention will be more readily apparent to those skilled in the art from a review of the detailed description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention can be determined from the appended claims.